Installation and Workover Control Systems (IWOCS) are used to provide hydraulic pressure and enable pressure and temperature monitoring during the installation and intervention of tubing hangers and subsea trees to enable production from subsea wells. They can also be used for injection (typically, using water) into wells to enhance or maintain production from other wells in the same field. Intervention tasks can include activities such as production flow testing, downhole well logging, and wirelining activities.
The tubing hanger, which suspends the production tubing (known as the downhole completion), may be installed in a subsea wellhead, a tubing head spool, or directly into a subsea tree. The mechanical tooling used in conjunction with the tubing hanger typically includes: a Tubing Hanger Running Tool (THRT), a Subsea Test Tree (SSTT), and a completion riser system. The THRT is hydraulically latched to the tubing hanger during installation or intervention. The SSTT, a self-contained valving arrangement, is affixed above the THRT (these two items are often kept assembled together). The completion riser system is attached above the SSTT and includes a series of tubular sections (known as “joints”) to provide a pressure containing conduit between the tubing hanger and the surface drilling rig. For safety reasons, during installation or intervention of a downhole completion it is necessary to employ a subsea Blow Out Preventer (BOP) stack with a drilling riser system connected back to the surface drilling rig. The BOP stack and drilling riser system for subsea applications includes an internal bore to enable passage of the tubing hanger, the THRT, the SSTT, and the completion riser (the bore of the BOP stack and the drilling riser system is usually 18 ¾″ minimum diameter). The drilling riser itself also includes a series of “joints” which are made up at surface, as required, to suit the specific water depth.
There are several variations of subsea trees currently being installed in field developments, one of which is a vertical tree whereby the drilling BOP stack is directly installed onto the tree re-entry mandrel. The trees may be installed directly onto an underwater wellhead or tubing head spool arrangement.
The system whereby the downhole completion has been installed through a BOP stack and the BOP stack installed on the tree has traditionally necessitated the use of two IWOCS umbilical systems and associated reels, connection, and attachment features. The first umbilical is installed inside the drilling riser and BOP stack and provides hydraulic control of the SSTT, the THRT, the tubing hanger, and downhole valves. This umbilical could be temporarily affixed to the completion riser during operations. An additional, umbilical external to the drilling riser would be incorporated for subsequent operation of the appropriate subsea tree valves and downhole valves for pressure testing, chemical injection, and monitoring. The external umbilical would typically be affixed to the drilling riser and would require an Emergency Disconnect Unit (EDU) retro-fitted to the BOP stack to permit the stack's Lower Marine Riser Package (LMRP) to be disconnected in the event of adverse weather conditions. The umbilical configuration typically includes direct hydraulic lines for shallow water applications and multiplex electro-hydraulic lines for deeper water applications.
Following satisfactory installation of the subsea tree onto the previously installed downhole completion (in the wellhead or tubing head spool) and flow testing of the well (if required), the internal tooling is retrieved. This includes the THRT, the SSTT, the completion riser system, and the internal umbilical. The external equipment, including the BOP stack, the drilling riser system, and the external umbilical would then be recovered back to the surface rig. Production of the formation fluids may then commence under planned operations. During the life of the well, various intervention and workover procedures may be necessary to ensure continued production. These procedures usually include the installation and subsequent retrieval of all the aforementioned equipment similarly to the procedures just discussed.